Resistor



C. A. NICKLE March 1, 1932.

RESISTOR Filed Aug. 9, 1929' Invenboh: Cli fomi A.Nickle,

His Aotowneg.

- CLIFFORD A. NICKLE,

Patented Mar. 1, 1932 UNITED STATES PATENT OFFICE.

RESISTOR Application filled August 9, 1929. Berlatlio. 884,756.

My invention relatesto electrical resistors and provides an improved resistor of the socalled self-reducing type, that is, one which has such a high negative temperature coeflioient that its initial resistance is reduced to a relatively low value when its temperature is raised as by the heating effect of current passing therethrough. The invention also provides an im roved method of making such resistors as we 1 as an improved terminal arrangement therefor.

Heretofore there has been a need for a satisfactory self-reducing resistor for use in motor starting circuits and the like where it is desired to provide a resistance of relatively high value when the circuit is initially closed, which resistance is automatically reduced after the initiation of current flow in the circuit. Thus, in starting electric motors, the

initial resistance of such a self-reducing re-- sistor serves to limit the starting or inrush current of the motor at standstill and thereafter the heating effect of the current serves automatically to decrease the resistance to a relatively low value after the motor is started. The simplicity of construction and operation of such an automaticself-reducing resistor current control arrangement obvious- 1y rendersit most desirable, if not ideal, for

motor starting and other similar service.

Heretofore various materials have been proposed for use as selfreducing resistors,

such, for example, as carbon, silicon, boron,

.-;copper oxide, magnetite, germanium, and varlous other substances and alloys.

However, as far as I am aware, all of the self-reducing resistors previously proposed have inherent deficiencies which render them more or less unsatisfactory for general commercial use. For example, many of theproposed self-reducing resistance materials are inconsistent in the res stance values obtained therewith even when the conditions for manufacture are under control. That is, the resistorunits even when produced as duplicates and in substantially the same manner have substantially different resistance vvalues as Well as different negative temperature coefiicients of resistance. Also the self-reducing resistance characteristics obtained with most OF SOEENECTADY, NEW YORK, ASBIGNC JB TO GENERAL ELECTRIC OOEPANY, A. CORPORATION OF NEW YORK of the proposed materials is not nearly of I it is impossible to produce a difference between the cold and hot resistance thereof of the magnitude required for practical .use. Also many of the proposed materials are so rare and expensive that general commercial use thereof in currentcontrolling service is prohibitive from the standpoint 0 cost alone.

The principal object of the present invention is to overcome the difliculties heretofore encountered and provide a self-reducing resistance that will withstand relatively high temperatures; that will give a change of large magnitude between the cold and the hot resistance values; that will give consistent resistance values and characteristics when manufactured and also after long continued service; and finally that may be manufactured cheaply enough to compete with other forms of current controlling devices now'in commercial use. 7

Briefly, in accordance with the present invention, these results are obtained by forming the self-reducing resistor of tellurium.

In its ordinary commercial form, tellurium is a metal powder of about 99% purity. I have found that this metal in solid form has a negative temperature coefiicient of re'sistance such as to give a negative change in resistance of the order of 10:1 over a'practical operating temperature range. Since the metal melts at around 452 deg. Ga a nelatively high maximum missible.

For certain service the. self-reducing retempe'rature limit is persistance elements may be formed by melting the powdered tellurium and casting itinto either a suitable container or into a self-sustaining term of resistance unit. However, I have found that in the self-sustaining cast form ordinarily the tellurium resistance units are too brittle and weak mechanically for general a plication. This is due probably to the fact t at the melted tellurium as it cools after casting crystallizes into relatively large crystals, thereby producing a relatively fragile structure. However, I have found this difiiculty may be overcome by. hot pressing the powdered tellurium into a self-sustaining solid form at a temperature around or somewhat below the actual melting temperature. The hot pressed tellurium has a micro-crystalline structure of substantially uniform resistance characteristic when manufactured under uniform temperature and pressure conditions. Furthermore, the micro-crystalline structure of the hot pressed body of tellurium provides such substantial mechanical strength that it not only is self-supporting and hence needs no container, but it is not easily broken by jars or shocks and will withstand even quite rough usage, such, for example, as dropping it on the floor. Moreover, the micro-crystalline tellurium possesses such a relatively high initial resistance value that solid units of large cross-sectional area may be employed, thus providing not only increased mechanical strength but also suflicient heat storage capacity to control the increase in temperature and thereby delay the automatic decrease in the self-reducing resistance for a. desired time interval after the flow of current therethrough is initiated.

, terminal connector, Fig. 2 shows schematical- 1y one way in which the powdered tellurium may be hot pressed into the self-reducing resistance units, such as shown in Fig. 1, Fig. 3 is a circuit diagram showing a plurality of the tellurium self-reducing resistor units connected in series in the starting circuit of a split phase type of alternating current induction motor, Figs. 4 and 5 are circuit diagrams showin the tellurium self-reducing resistors applie for starting three-phase alternating current induction motors, and Fig. 6 shows the resistors connected ina tungsten lamp circuit for controlling the inrush current.

The self-reducing resistor unit 10 shown in Fig. 1 is formed of tellurium hot pressed into a solid. rodin the manner described in connection with Fig. 2.

The tellurium resistance unit 10 is provided with the screw-threaded terminals 11 and 12, preferably formed of iron and having suitable nuts 13 thereon for securing the circuit conductors to the resistance unit and establishing good electrical contact therewith.

While the terminal construction just described is preferable, terminals also may be provided by electroplating the ends of the resistor unit or by hot pressing other good conducting material, such as iron powder, into the ends of the resistor unit. For the larger size units, the terminals may be clamped under pressure around the ends of the tellurium rod.

Since the resistance of the tellurium is relatively .high, being of the order of 2,000,000 ohms per mil foot at normal room temperature, the cross section of the resistance unit may be relatively large. This perinitsa unit of substantial size and mechanical strength to be employed. even where comparatively high resistance Values are d sired.

The crosssectional hape of the resistor 10 depends upon the particular form of the mold in which it is hot pressed, and may be round, square, or any other desired shape. ever, the resistor 10 preferably is of circular cross section when made in the manner illustrat d in Fig. 2.

In this figure the mold 14 may be formed of fused quartz so as to, withstand high temperatures, and is provided with a slightly tapering cylindrical bore 15 for receiving the powdered tellurium to be hot pressed. The fused quartz mold. is better than a mold formed of metal, for the reason that it does not tend to alloy or adhere to the powdered tellurium during the hot pressing process. I

The mold 14 rests upon a base 16 which also may be formed of fused quartz with the tapering bore 15 increasing in diameter from the top tothe bottom. The iron terminal screw 12 is placed in a suitable recess formed in the base 16 and extends partially into the bore 15 so as to be embedded in the end of the tellurium unit as it is hot pressed. The screw terminal 11 is located in a similar manner in the plunger 17 by means of which pressure is applied to the powdered tellurium during the ot pressing process.

The bore 15 is then filled with powdered tellurium with the mold heated by any suitable means, not shown, to a temperature around or somewhatjbelow the melting temperature of the tellurium. I have found that the recise temperature employed is not criticalut may be anywhere within the range from just below the melting point to the temerature at which the tellurium powder just egins to soften.

Pressure then is applied to the plunger 17 with the heated tellurium in a plastic state. As a result, the volume of the powdered tellurium is reduced byabout one-half, as indicated by thedotted lines in Fig. 2. During the compression of the lastic tellurium the terminal screws 11 an 12 are firmly embedded into' the ends of themesistor unit. Furthermore, I have found that the embedded ends of the iron terminals are thor- Hownausea oughly wetted by the plastic tellurium, thus establishing a ood current conducting relationship there etween. I have found that the iron terminal screws are the most satisfactory, althou h other forms of terminals may be employe if desired.

The base 16 is then removed and the hot pressed tellurium resistor unit is readily orced out of the bottom of the tapered bore 15in the solid mold 14. Due to the tapering shape of the bore 15, the tendency of the nary orms of split molds.

' structure obtained and in addition insures re-- The tellurium resistor unit formed in the above manner is mechanically strong even when removed hot from the mold, and has a ve% fine micro-crystalline grain structure.

hile I have obtained the best results by hot pressing the powdered tellurium in the manner just described, the tellurium resistor units also may be made by casting the melted tellurium in a hollow container of suitable shape or by cold pressing the powdered metal 'un er heavy pressure and then sintering the compressed blllets. Ihave found, however, that the hot pressing imparts greater mechanical strength due to the micro-crystalline sistor units of precisely the desired she with a nicely finished surface. Also, the application of the resistor terminals is facilitated by the hot pressing method.

Fig. 3 shows the self-reducing tellurium resistor units employed in the starting circuit of a split phase type of alternating current induction motor, such as used in domesticelectric refrigerators. The motor has a runnin winding 20 and a's tarting winding 21, hot 1 acting upon the squirrel cage rotor 22. The ener izmg circuit-of the motor is controlled by 51c switch mechanism 23 which may be of the form described and claimed in the Hutt Patent 1,637,042 having a temperature restponsive operating element 24 and an overloa protective element 25 and a manualv operating element 26. The self-reducing tellurium resistor 10 is connected. in series winding 21, which 10. The articular function of the self-reducing te urium resistor in this service is to com ensate for variations in the line voltage. hus, with normal or high voltage, the self-reducing tellurium resistors serve to limit the current supplied to the starting winding to a permissib e value such asto avoid flickering in the lighting circuit on which the electric refrigerator ma be used. a

In case of low vo tage such that the motor does not readily start with the limited current through the starting winding, the selfreducing tellurium resistors automatically decrease their resistance due to the he'atin efi'ect ofthe current flow therethrough unt1 sufiicient current is obtained to insure'starting of the motor. This automatic reduction of the resistors 10 occurs before the thermal overload element 25 has time to function. Thus, with the self-reducing tellurium resistors of the present inventlon the starting of the refrigerator motor is assured under all voltage conditions, and at the same time excessive starting currents on normal or high voltage such as to flicker the lights are effectively avoided.

In Fig. 4, the self-reducing tellurium resistors 10 are connected in each line of the' three phase induction motor 30 and serve to reduce the voltage a plied to the motor 30 in starting to a desire value. As the motor starts and reaches runnin speed, the heating efiect of the current flowing through the resistors 10 causes'the initial resistance thereof practically to disappear and thereby impress substantially full line voltage on the motor 30.

In Fig. 5, the'self-reducing tellurium resistorslO are connected in the secondary or rotor circuit of a three-phase wound rotor type of induction motor 31. Thus,'in startin the resistors 10 are of relatively high value to provide a good starting torque, and as the motor starts and comes up to speed the. resistance is decreased to provide efiicient o eration with normal running torque.

In ig. 6, the self-reducing tellurium resistorlO is connected in circuit with a rality of tun en lam s 32 in order to compensate for t e marke positive temperature coeflicient of these devices. Upon closing the the current reduces the resistor 10 and at the same time increases the resistance of the lamps to thefull value.

It will be apparent to those sk1lled in the art that my improved form of self-reducing resistor maybe employed with advantage in many other fields of service. i

I What I claimas newand desire to secure.

lbs

plus-.

by Letters Patentof the United States, ist-' 1. A self-reducingresi 's tance for electric circuits composed of micro-crystalline tellurium.

2. A self-reducing resistor for electric cirhand this 8th cuits comprising tellurium formed micro-crystalline solid body.

3. A self-reducing resistance for electric circuits consisting of a, self-sustaining body of micro-crystalline tellurium.

4; A resistor unit composed of a mass of tellurium formed by pressure and heat into a micro-crystalline solid body.

into a.

5. A resistor unit consisting of a self-sus taining rod of micro-crystalline tellurium with metallic terminals embedded in the ends thereof.

6. The method of forming tellurium resistor units consisting of heatlng a, powdered mass of tellurium to a temperature below the melting point and applying pressure to solidify the mass.

7. The method of formin resistor units consisting of embeddin solid metallic terminals in a mass of, powdered tellurium, heating the mass to a temperature at which the tellurium wets the terminals, and applying pressure to solidify the mass about the terminals.

In witness whereof, I have hereunto set my day of August, 1929. CLIFFORD A. NICKLE. 

